When making advanced semiconductor devices, copper interconnects may offer a number of advantages over those made from aluminum. For that reason, copper has become the material of choice for making such devices' interconnects. As device dimensions shrink so does conductor width—leading to higher resistance and current density. Increasing current density can increase the rate at which copper atoms are displaced when current passes through a copper conductor. Such electromigration can cause vacancies, which may lead to voids. Those voids may form at the interface between the copper conductor and a barrier layer that is formed on the conductor. If a void grows to a size that creates metal separation, e.g., near a via that contacts the conductor, it may cause an open-circuit failure.
One way to prevent electromigration from causing interconnect failure is to limit the amount of current that passes through the conductor. That solution to the electromigration problem is impractical, however, because devices will operate at progressively higher currents, even as they continue to shrink. As an alternative, reliability can be enhanced by slowing metal diffusion along the fastest diffusion path—i.e., along the copper/barrier layer interface. Applying various surface treatments, e.g., exposing the copper layer to ammonia and/or silane prior to forming the barrier layer on the copper layer, may reduce metal diffusion along that interface. Introducing dopants into the copper layer may also limit diffusion. Unfortunately, these techniques for reducing the rate at which copper diffuses may raise the resistance of the copper layer significantly.
Accordingly, there is a need for an improved process for making a semiconductor device that includes copper interconnects. There is a need for such a process that reduces electromigration without significantly raising conductor resistance. The method of the present invention provides such a process.